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Y-TZP에서 표면 처리에 따른 저온열화 거동의 직접적 관찰
정태주,김혜성,Chung, Tai-Joo,Kim, Hye-Sung 한국분말야금학회 2010 한국분말재료학회지 (KPMI) Vol.17 No.3
Low temperature degradation behavior in yttria stabilized tetragonal zirconia polycrystal (Y-TZP) ceramics was microscopically observed from the phase contrast between monoclinic surface and tetragonal matrix. The degradation behavior was dependent on the surface treatment of sintered Y-TZP, even if the sintering history is same. In the mirror polished specimen, the monoclinic layer appeared in a uniform thickness from the surface. On the contrary, for the specimen with coarse scratch, the thickness of degraded surface was more than double especially from the coarse scratch. Since the scratch results in local deformation, the residual stress should be induced around the scratch. With the transformation from tetragonal to monoclinic, the volume expansion exerts a stress on a neighboring grains and promotes a successive phase transformation. Such a autocatalytic effect can be triggered from the part of coarse scratch.
입자크기와 열처리 분위기 변화에 따른 Y-TZP에서의 상안정성 변화
정태주,안승수,송은화,오경식,이종숙,김영식,Chung, Tai-Joo,Ahn, Seung-Su,Song, Eun-Wha,Oh, Kyung-Sik,Lee, Jong-Sook,Kim, Young-Sik 한국분말야금학회 2006 한국분말재료학회지 (KPMI) Vol.13 No.5
The phase stability of tetragonal phase in Y-TZP was investigated in terms of the distribution of grain sizes and heat-treating atmosphere. Y-TZP with various grain sizes were prepared using duration time at $1600^{\circ}C$ as experimental parameter. Accumulated grain size distributions were built from the SEM micrographs and the amount of tetragonal phase were measured using XRD. Both results were compared to determine the critical grain size before and after heat-treatment in vacuum. The critical grain size drastically decreased compared with the small increase of average grain size due to the autocatalytic effect which critically affects the tetragonal to monoclinic phase transformation. After heat-treatment in reductive atmosphere critical grain size relatively increased due to the stabilization of tetragonal phase. The formation of oxygen vacancies during heat-treatment was ascribed to the increase of stability.
WC와 Co원료 입자크기 변화에 따른 WC-Co계 초경합금의 특성 변화
정태주,안선용,백용균,Chung, Tai-Joo,Ahn, Sun-Yong,Paek, Yeong-Kyeun 한국세라믹학회 2005 한국세라믹학회지 Vol.42 No.3
서로 다른 입자크기를 갖는 WC와 Co 분말 원료를 사용하여 $WC-10\;wt\%$Co 초경합금을 제조하였다. 이로부터 WC와 Co 원로. 입자크기가 제조된 초경합금의 성질에 미치는 영향에 대해 고찰하였다. WC 원료 입자크기가 클수록 제조된 초경합금이 파괴인성이 높고 경도는 낮게 나타나는데, 이러한 경향은 Co 원료 입자크기에 크게 영향 받지 않음을 알 수 있었다. Co 원료 크기의 영향 외에도 Co 원료 크기가 초경합금의 특성에 영향을 주는 것으로 밝혀졌는데, 동일한 WC 원료를 사용하여도 미세한 Co 원료를 사용할 경우, 보다 조대한 WC를 함유하여 인성이 우수한 초경합금을 제조할 수 있었다. 이로부터 Co 원료 입자크기가 초경합금의 미세구조 및 그 특성에 중요한 역할을 함을 알 수 있었다 The effect of variation in particle size of WC and Co powder on the properties of WC-Co alloys was investigated. WC and Co powders having different particle sizes were used in the fabrication of $WC-10\;wt\%$Co composites. High hardness and low fracture toughness alloy was obtained with the decrease in WC particle size regardless of Co particle size. It was newly found in this investigation that the initial particle size of Co as well as WC had a great role in the microstructure and properties of WC-Co hard materials. The average grain size and fracture toughness of WC-Co alloys using same WC powder size increased and their hardness decreased with the use of relatively finer Co binder.
WC 첨가 방법에 따른 Ti(CN)-Co/Ni 계 서멧트의 미세조직 및 특성변화
정태주,안선용,안승수,신명수,김학규,김경배,오경식,이혁재,Chung, Tai-Joo,Ahn, Sun-Yong,Ahn, Seung-Su,Shin, Myung-Soo,Kim, Hak-Kyu,Kim, Kyung-Bae,Oh, Kyung-Sik,Lee, Hyuk-Jae 한국분말야금학회 2007 한국분말재료학회지 (KPMI) Vol.14 No.2
In the Ti(CN)-Co/Ni cermet, WC is an effective additive for increasing sinterability and mechanical properties such as toughness and hardness. In this work, WC, (WTi)C and (WTi)(CN) were used as the source of WC and their effects were investigated in the respect of microstructural evolution and mechanical properties. Regardless of the kinds of WC sources, the hard phase with dark core and bright rim structure was observed in the Ti(CN)-Co/Ni cermet under the incorporation of relatively small amount of WC. However, hard phases with bright core began to appear and their frequency increased with the increase of all kinds of WC source addition. The ratio of bright core to dark one in the (TiW)(CN)-Co/Ni cermet was greatest under the incorporation of (WTi)C compared at the same equivalent amount of WC. The mechanical properties were improved with the addition of WC irrespective of the kinds of sources, but the addition of (WTi)(CN) was less effective for the increase of fracture toughness.
LSGM계 음극지지형 고체산화물 연료전지에 적용된 LDC 완충층의 효과
송은화,정태주,김혜령,손지원,김병국,이종호,이해원,Song, Eun-Hwa,Chung, Tai-Joo,Kim, Hae-Ryoung,Son, Ji-Won,Kim, Byung-Kook,Lee, Jong-Ho,Lee, Hae-Weon 한국세라믹학회 2007 한국세라믹학회지 Vol.44 No.12
LSGM$(La_{0.8}Sr_{0.2}Ga_{0.8}Mg_{0.2}O_{3-{\delta}})$ is the very promising electrolyte material for lower-temperature operation of SOFCs, especially when realized in anode-supported cells. But it is notorious for reacting with other cell components and resulting in the highly resistive reaction phases detrimental to cell performance. LDC$(La_{0.4}Ce_{0.6}O_{1.8})$, which is known to keep the interfacial stability between LSGM electrolyte and anode, was adopted in the anode-supported cell, and its effect on the interfacial reactivity and electrochemical performance of the cell was investigated. No severe interfacial reaction and corresponding resistive secondary phase was found in the cell with LDC buffer layer, and this is due to its ability to sustain the La chemical potential in LSGM. The cell exhibited the open circuit voltage of 0.64V, the maximum power density of 223 $mW/cm^2$, and the ohmic resistance of $0.17{\Omega}cm^2$ at $700^{\circ}C$. These values were much improved compared with those from the cell without any buffer layer, which implies that formation of the resistive reaction phases in LSGM and then deterioration of the cell performance is resulted mainly from the La diffusion from LSGM electrolyte to anode.
과립형 β-삼인산칼슘을 사용한 Brushite계 골시멘트의 보관 조건에 따른 경화 특성
이선애,정태주,오경식,Lee, Sun-Ae,Chung, Tai-Joo,Oh, Kyung-Sik 한국세라믹학회 2008 한국세라믹학회지 Vol.45 No.10
In the bone cement composed of dense granules of $\beta-Ca_3(PO_4){_2}(\beta-TCP)$ and $Ca(H_2PO_4){_2}H_2O$, the compressive strength, setting time and temperature rise were measured to observe the degradation of cement with respect to the stored days before setting. Decreases of compressive strength and temperature rise were observed, while setting time increased with respect to the stored days. The similar trends were repeated with the increase of temperature of storage. Such a change virtually meant the fading of the character of cement and it took place only when the two starting materials were mixed during storage. The degradation could be mitigated taking advantage of granular $\beta$-TCP instead of powdery one. The formation of $CaHPO_4$, which resulted from reaction with ambient humidity, was attributed to the degradation observed during storage. Dependence of the degradation behavior on mixing and temperature during storage was discussed in terms of the driving force for reaction of cement.
In-situ 고온 딜라토미터를 이용한 탄화붕소의 소결거동 연구
이혁재,김범섭,정태주,Lee, Hyukjae,Kim, Bum-Sup,Chung, Tai-Joo 한국분말야금학회 2014 한국분말재료학회지 (KPMI) Vol.21 No.2
A high temperature dilatometer attached to a graphite furnace is built and used to study the sintering behavior of $B_4C$. Pristine and carbon doped $B_4C$ compacts are sintered at various soaking temperatures and their shrinkage profiles are detected simultaneously using the dilatometer. Carbon additions enhance the sinterability of $B_4C$ with sintering to more than 97% of the theoretical density, while pristine $B_4C$ compacts could not be sintered above 91% due to particle coarsening. The shrinkage profiles of $B_4C$ reveal that the effect of carbon on the sinterability of $B_4C$ can be seen mostly below $1950^{\circ}C$. The high temperature dilatometer delivers very useful information which is impossible to obtain with conventional furnaces.
LSGM계 전해질 지지형 고체산화물 연료전지의 특성평가
송은화,김광년,정태주,손지원,김주선,이해원,김병국,이종호,Song, Eun-Hwa,Kim, Kwang-Nyeon,Chung, Tai-Joo,Son, Ji-Won,Kim, Joo-Sun,Lee, Hae-Weon,Kim, Byung-Kook,Lee, Jong-Ho 한국세라믹학회 2006 한국세라믹학회지 Vol.43 No.5
LSGM(($La_xSr_{1-x})(Ga_yMg_{1-y})O_3$) electrolyte is known to show very serious interfacial reaction with other unit cell components, especially with an anode. Such an interfacial reaction induced the phase instability of constituent component and deterioration of the unit cell performance, which become the most challenging issues in LSGM-based SOFCs. In this study, we fabricated LSGM($La_{0.8}Sr_{0.2}Ga_{0.83}Mg_{0.17}O_x$) electrolyte supported-type cell in order to avoid such interfacial problem by lowering the heat-treatment temperature of the electrode fabrication. According to the microstructural and phase analysis, there was no serious interfacial reaction at both electrolyte/anode and electrolyte/cathode interfaces. Moreover, from the electrochemical characterization of the unit cell performance, there was no distinct deterioration of the open cell voltage as well as an internal cell resistance. These results demonstrate the most critical point to be concerned in LSGM-based SOFC is either to find a proper electrode material which will not give any interfacial reaction with LSGM electrolyte or to properly adjust the processing variables for unit cell fabrication, to reduce the interfacial reaction.
나노 분말을 Spark Plasma 소결해 제조한 PbTe의 열전 특성
전은영,김호영,김참,오경식,정태주,Jun, Eun-Young,Kim, Ho-Young,Kim, Cham,Oh, Kyung-Sik,Chung, Tai-Joo 한국분말야금학회 2018 한국분말재료학회지 (KPMI) Vol.25 No.5
Nanoparticles of PbTe are prepared via chemical reaction of the equimolar aqueous solutions of $Pb(CH_3COO)_2$ and Te at $120^{\circ}C$. The size of the obtained particles is 100 nm after calcination in a hydrogen atmosphere. Dense specimens for the thermoelectric characterization are produced by spark plasma sintering of prepared powders at $400^{\circ}C$ to $500^{\circ}C$ under 80 MPa for 5 min. The relative densities of the prepared specimens reach approximately 97% and are identified as cubic based on X-ray diffraction analyses. The thermoelectric properties are evaluated between $100^{\circ}C$ and $300^{\circ}C$ via electrical conductivity, Seebeck coefficient, and thermal conductivity. Compared with PbTe ingot, the reduction of the thermal conductivities by more than 30% is verified via phonon scattering at the grain boundaries, which thus contributes to the increase in the figure of merit.
NaBr 및 NaF 용액에 대한 Sn-3.0Ag-0.5Cu 솔더 합금의 Electrochemical Migration 특성
정자영,장은정,유영란,이신복,김영식,주영창,정태주,이규환,박영배,Jung, Ja-Young,Jang, Eun-Jung,Yoo, Young-Ran,Lee, Shin-Bok,Kim, Young-Sik,Joo, Young-Chang,Chung, Tai-Joo,Lee, Kyu-Hwan,Park, Young-Bae 대한용접접합학회 2007 대한용접·접합학회지 Vol.25 No.3
Electrochemical migration characteristics of Pb-free solder alloys are quantitatively correlated with corrosion characteristics in harsh environment conditions. In-situ water drop test and corrosion resistance test for Sn-3.0Ag-0.5Cu solder alloys were carried out in NaBr and NaF solutions to obtain the electrochemical migration lifetime and pitting potential, respectively. Sn-3.0Ag-0.5Cu solder alloy shows similar ionization and electrochemical migration behavior with pure Sn because of Ag and Cu do not migrate due to the formation of resistant intermetallic compounds inside solder itself. Electrochemical migration lifetime in NaBr is longer than in NaF, which seems to be closely related to higher pitting potential in NaBr than NaF solution. Therefore, it was revealed that electrochemical migration lifetime of Sn-3.0Ag-0.5Cu solder alloys showed good correlation to the corrosion resistance, and also the initial ionization step at anode side is believed to be the rate-determining step during electrochemical migration of Pb-free solders in these environments.